Evolution of Flowers

While land plants have existed for about 425 million years, the first ones reproduced by a simple adaptation of their aquatic counterparts: spores. In the sea, plants—and some animals—can simply scatter out genetic clones of themselves to float away and grow elsewhere. This is how early plants reproduced. But plants soon evolved methods of protecting these copies to deal with drying out and other abuse which is even more likely on land than in the sea. The protection became the seed, though it had not yet evolved the flower. Early seed-bearing plants include the ginkgo and conifers. The earliest fossil of a flowering plant, Archaefructus liaoningensis, is dated about 125 million years old.[6] Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin. Recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps.

Recent DNA analysis (molecular systematics) shows that Amborella trichopoda, found on the Pacific island of New Caledonia, is the sister group to the rest of the flowering plants, and morphological studies suggest that it has features which may have been characteristic of the earliest flowering plants.

The general assumption is that the function of flowers, from the start, was to involve other animals in the reproduction process. Pollen can be scattered without bright colors and obvious shapes, which would therefore be a liability, using the plant's resources, unless they provide some other benefit. One proposed reason for the sudden, fully developed appearance of flowers is that they evolved in an isolated setting like an island, or chain of islands, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example), the way many island species develop today. This symbiotic relationship, with a hypothetical wasp bearing pollen from one plant to another much the way fig wasps do today, could have eventually resulted in both the plant(s) and their partners developing a high degree of specialization. Island genetics is believed to be a common source of speciation, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Note that the wasp example is not incidental; bees, apparently evolved specifically for symbiotic plant relationships, are descended from wasps.

Likewise, most fruit used in plant reproduction comes from the enlargement of parts of the flower. This fruit is frequently a tool which depends upon animals wishing to eat it, and thus scattering the seeds it contains.

While many such symbiotic relationships remain too fragile to survive competition with mainland animals and spread, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life.

While there is only hard proof of such flowers existing about 130 million years ago, there is some circumstantial evidence that they did exist up to 250 million years ago. A chemical used by plants to defend their flowers, oleanane, has been detected in fossil plants that old, including gigantopterids[10], which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail; one of the earliest examples of petrification.

The similarity in leaf and stem structure can be very important, because flowers are genetically just an adaptation of normal leaf and stem components on plants, a combination of genes normally responsible for forming new shoots. The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior".

Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that many of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection.

Symbolisms of Flowers

Many flowers have important symbolic meanings in Western culture. The practice of assigning meanings to flowers is known as floriography. Some of the more common examples include:

• Red roses are given as a symbol of love, beauty, and passion.
• Poppies are a symbol of consolation in time of death. In the UK, New Zealand, Australia and Canada, red poppies are worn to commemorate soldiers who have died in times of war.
• Irises/Lily are used in burials as a symbol referring to "resurrection/life". It is also associated with stars (sun) and its petals blooming/shining.
• Daisies are a symbol of innocence.

Flowers within art are also representative of the female genitalia, as seen in the works of artists such as Georgia O'Keeffe, Imogen Cunningham, Veronica Ruiz de Velasco, and Judy Chicago, and in fact in Asian and western classical art. Many cultures around the world have a marked tendency to associate flowers with femininity.

The great variety of delicate and beautiful flowers has inspired the works of numerous poets, especially from the 18th-19th century Romantic era. Famous examples include William Wordsworth's I Wandered Lonely as a Cloud and William Blake's Ah! Sun-Flower.

Because of their varied and colorful appearance, flowers have long been a favorite subject of visual artists as well. Some of the most celebrated paintings from well-known painters are of flowers, such as Van Gogh's sunflowers series or Monet's water lilies. Flowers are also dried, freeze dried and pressed in order to create permanent, three-dimensional pieces of flower art.

The Roman goddess of flowers, gardens, and the season of Spring is Flora. The Greek goddess of spring, flowers and nature is Chloris.

In Hindu mythology, flowers have a significant status. Vishnu, one of the three major gods in the Hindu system, is often depicted standing straight on a lotus flower. Apart from the association with Vishnu, the Hindu tradition also considers the lotus to have spiritual significance. For example, it figures in the Hindu stories of creation.

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Flowers Morphology

Flowering plants are heterosporangiate, producing two types of reproductive spores. The pollen (male spores) and ovules (female spores) are produced in different organs, but the typical flower is a bisporangiate strobilus in that it contains both organs.

A flower is regarded as a modified stem with shortened internodes and bearing, at its nodes, structures that may be highly modified leaves. In essence, a flower structure forms on a modified shoot or axis with an apical meristem that does not grow continuously (growth is determinate). Flowers may be attached to the plant in a few ways. If the flower has no stem but forms in the axil of a leaf, it is called sessile. When one flower is produced, the stem holding the flower is called a peduncle. If the peduncle ends with groups of flowers, each stem that holds a flower is called a pedicel. The flowering stem forms a terminal end which is called the torus or receptacle. The parts of a flower are arranged in whorls on the torus. The four main parts or whorls (starting from the base of the flower or lowest node and working upwards) are as follows:

• Calyx: the outer whorl of sepals; typically these are green, but are petal-like in some species.
• Corolla: the whorl of petals, which are usually thin, soft and colored to attract insects that help the process of pollination.
• Androecium (from Greek andros oikia: man’s house): one or two whorls of stamens, each a filament topped by an anther where pollen is produced. Pollen contains the male gametes.
• Gynoecium (from Greek gynaikos oikia: woman’s house): one or more pistils. The female reproductive organ is the carpel: this contains an ovary with ovules (which contain female gametes). A pistil may consist of a number of carpels merged together, in which case there is only one pistil to each flower, or of a single individual carpel (the flower is then called apocarpous). The sticky tip of the pistil, the stigma, is the receptor of pollen. The supportive stalk, the style becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma, to the ovules, carrying the reproductive material.

Although the floral structure described above is considered the "typical" structural plan, plant species show a wide variety of modifications from this plan. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species. For example, the two subclasses of flowering plants may be distinguished by the number of floral organs in each whorl: dicotyledons typically having 4 or 5 organs (or a multiple of 4 or 5) in each whorl and monocotyledons having three or some multiple of three. The number of carpels in a compound pistil may be only two, or otherwise not related to the above generalization for monocots and dicots.

In the majority of species individual flowers have both pistils and stamens as described above. These flowers are described by botanists as being perfect, bisexual, or hermaphrodite. However, in some species of plants the flowers are imperfect or unisexual: having only either male (stamens) or female (pistil) parts. In the latter case, if an individual plant is either female or male the species is regarded as dioecious. However, where unisexual male and female flowers appear on the same plant, the species is considered monoecious.

Additional discussions on floral modifications from the basic plan are presented in the articles on each of the basic parts of the flower. In those species that have more than one flower on an axis—so-called composite flowers—the collection of flowers is termed an inflorescence; this term can also refer to the specific arrangements of flowers on a stem. In this regard, care must be exercised in considering what a ‘‘flower’’ is. In botanical terminology, a single daisy or sunflower for example, is not a flower but a flower head—an inflorescence composed of numerous tiny flowers (sometimes called florets). Each of these flowers may be anatomically as described above. Many flowers have a symmetry, if the perianth is bisected through the central axis from any point, symmetrical halves are produced—the flower is called regular or actinomorphic, e.g. rose or trillium. When flowers are bisected and produce only one line that produces symmetrical halves the flower is said to be irregular or zygomorphic. e.g. snapdragon or most orchids.

Flowers Specialization & Pollination

Each flowers have a specific design which best encourages the transfer of its pollen. Cleistogamous flowers are self pollinated, after which, they may or may not open. Many Viola and some Salvia species are known to have these types of flowers.

Entomophilous flowers attract and use insects, bats, birds or other animals to transfer pollen from one flower to the next. Flowers commonly have glands called nectaries on their various parts that attract these animals. Some flowers have patterns, called nectar guides, that show pollinators where to look for nectar. Flowers also attract pollinators by scent and color. Still other flowers use mimicry to attract pollinators. Some species of orchids, for example, produce flowers resembling female bees in color, shape, and scent. Flowers are also specialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate). In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas—arranged with equally pointed precision—of all of the flowers it visits.

Anemophilous flowers use the wind to move pollen from one flower to the next, examples include the grasses, Birch trees, Ragweed and Maples. They have no need to attract pollinators and therefore tend not to be "showy" flowers. Male and female reproductive organs are generally found in separate flowers, the male flowers having a number of long filaments terminating in exposed stamens, and the female flowers having long, feather-like stigmas. Whereas the pollen of entomophilous flowers tends to be large-grained, sticky, and rich in protein (another "reward" for pollinators), anemophilous flower pollen is usually small-grained, very light, and of little nutritional value to insects.